There are two answers to your question. First, the most expensive off-the-rack item (meaning, ready-made, or something you can just buy from the store), second, the most expensive item that can be made (meaning, custom-made, or custom-order, cannot be bought from the store).
For most expensive off-the-rack item, it is probably Hermes. Other contenders are brands such as Goyard, Louis Vuitton, Valextra (the Italian version of Hermes), Asprey, etc..
The most expensive item that can be made is really hard to say. For all these brands (Hermes, Vuitton, Goyard, Valextra, etc.) you can order anything you create, and the price could be as high as you can imagine. You can custom-order a crocodile bag from Hermes, or ask Asprey to create a bag with a pear-size diamond for millions. There are also other smaller makers who charge exorbant amount for custom-order bags. "April In Paris", a shop in San Francisco (
http://www.aprilinparis.us/home), is run by a lady who formerly worked in Hermes. She will create any handbag for you that would cost higher than Hermes.
For the really rich and famous (or, in old-fashioned term, the aristocrats), they usually don't buy items that are off-the-rack, because they run the risk of carrying the same item as someone else (imagine if Queen Elizabeth were to carry the same bag as you if she bought it from a shop!). They always have their items custom-made for them, and the prices for these unique items are quite secretive - you won't read about them in newspapers or magazines.